This invention relates to a method and apparatus for separating air. The most important method commercially of separating air is by rectification. The most frequently used air separation cycles include the steps of compressing a stream of air, purifying the resulting stream of compressed air by removing water vapour and carbon dioxide, and cooling the stream of compressed air by heat exchange with returning product streams to a temperature suitable for its rectification. The rectification is performed in a so-called "double rectification column" comprising a higher pressure and a lower pressure rectification column, i. e. one of the two columns operates at higher pressure than the other. Most if not all of the air is introduced into the higher pressure column and is separated into oxygen-enriched liquid air and nitrogen vapour. The nitrogen vapour is condensed. A part of the condensate is used as liquid reflux in the higher pressure column. Oxygen-enriched liquid is withdrawn from the bottom of the higher pressure column, is sub-cooled and is introduced into an intermediate region of the lower pressure column through a throttling or pressure reduction valve. The oxygen-enriched liquid is separated into substantially pure oxygen and nitrogen products in the lower pressure column. These products are withdrawn in the vapour state from the lower pressure column and form the returning streams against which the incoming air stream is heat exchanged. Liquid reflux for the lower pressure column is provided by taking the remainder of the condensate from the higher pressure column, sub-cooling it, and passing it into the top of the lower pressure column through a throttling or pressure reduction valve.
Conventionally, the lower pressure column is operated at pressures in the range of 1 to 1.5 atmospheres absolute. At such pressures, it is desirable to link the higher and lower pressure columns by using liquid oxygen at the bottom of the lower pressure column to meet the condensation duty at the top of the higher pressure column. Accordingly, nitrogen vapour from the top of the higher pressure column is heat exchanged with liquid oxygen in the bottom of the lower pressure column. Sufficient liquid oxygen is able to be evaporated thereby to meet the requirements of the lower pressure column for reboil and to enable a good yield of gaseous oxygen product to be achieved. The pressure at the top of the higher pressure column and hence the pressure to which the incoming air is compressed is arranged to be such that the temperature of the condensing nitrogen is a degree or two Kelvin higher than that of the boiling oxygen in the lower pressure column.
Many commercial processes use oxygen containing less than one percent by volume of impurities. There are however some processes, for example, coal gasification, which desirably use oxygen of lower purity, typically containing from 3 to 20% by volume of impurities. U.S. Pat. No. 4 410 343 (Ziemer) discloses that when such lower purity oxygen is required, rather have the above described link between the lower and higher pressure columns, air is employed to boil oxygen in the bottom of the lower pressure column in order both to provide reboil for that column and to evaporate the oxygen product. The resulting condensed air is then fed into both the higher pressure and the lower pressure columns. A stream of oxygen-enriched liquid is withdrawn from the higher pressure column, is passed through a throttling valve and a part of it is used to perform the nitrogen condensing duty at the top of the higher pressure column. U.S. Pat. No. 3 210 951 also discloses a process for producing impure oxygen in which air is employed to boil oxygen in the bottom of the lower pressure column in order both to provide reboil for that column and to evaporate the oxygen product. In this instance, however, oxygen-enriched liquid from an intermediate region of the lower pressure column is used to fulfil the duty of condensing nitrogen vapour produced in the higher pressure column.
It is known to take the stream of air for separation from an air compressor forming part of a gas turbine. This practice typically entails operating the higher pressure column at substantially the same pressure as the outlet pressure of the compressor. Typically, such air compressors operate at a pressure of from 10 to 20 atmospheres. If the higher pressure column is operated at such a pressure, there is a concomitant increase in the pressure at which the lower pressure column is operated.
In theory, operating the lower pressure column at a higher pressure than one in the range of 1 to 1.5 atmospheres absolute is advantageous since it reduces the effect of pressure drop in the heat exchanger in which the compressed air stream is cooled by heat exchange with the product streams. One consequence, however, of operating the lower pressure rectification column at a higher pressure is, according to our analysis, that the demand for liquid nitrogen reflux in the higher pressure column thereby tending to starve the lower pressure column of liquid nitrogen reflux while at the same time the demand for liquid nitrogen in the lower pressure rectification column also increases. As a result, a further consequence is that the liquid air stream fed to the higher pressure rectification column then has a greater adverse impact on the performance of the air separation process.